Method and apparatus for collecting and then disposing sand contained in production fluid

ABSTRACT

Production fluid mixture is directed through a sand removal unit ( 12 ) in a retrievable module ( 1 ) installed in a modular seabed processing system, and the unit ( 12 ) collects sand from the mixture. The production fluid mixture then enters a fluid separation chamber ( 17 ) in the module ( 1 ) where it is separated into a gas and a mixture of oil and water. Seawater is drawn in from outside the module and is pumped into the sand removal unit ( 12 ) to flush the collected sand. The mixed flushed sand and seawater is commingled with the mixture of oil and water downstream of the fluid separation chamber ( 17 ) and the commingled mixture is conveyed to a host facility. There, the sand is separated from the commingled mixture and is disposed of.

The present invention relates to a method and apparatus for collecting sand contained in production fluid from a hydrocarbon reservoir, and disposing of the collected sand.

Production fluid from a hydrocarbon reservoir may carry sand. This sand causes erosional damage to valves and other processing equipment that the production fluid passes through. If the velocity of the production fluid falls below a specific speed as a result of, say, reduced production rates, entering processing vessels such as separators or an increase in the diameter of piping, the production fluid is likely to deposit sand which can effect equipment Progressive sand deposition in a separator vessel reduces the available processing volume of the vessel leading to poor separation performance. Sand may be deposited in the cavities of valves which may stop them from closing. It may also damage valve seals resulting in leakage. Furthermore, sand may erode the internal components of a pump causing leakage and eventual failure of the pump.

One technique of overcoming the problem of sand in production fluid is to install a sand screen in each wellbore. The sand screen is reasonably effective but does not stop all the sand in the production fluid. After a period of time, the sand screen can fail. Such failure requires expensive work on the well to rectify which causes a loss of production.

There may also be provided a sand removal vessel upstream of any equipment that will be susceptible to damage from sand in the production fluid. However, the sand removal vessel will eventually fill with sand. Hence, collected sand will need to be regularly removed from the vessel. This may be done by recovering the sand removal vessel to a surface ship that is certified to handle hydrocarbon fluids where the sand is removed for cleaning. The sand may then be deposited locally. Alternatively, the sand removal vessel may be taken to shore where the sand is removed, cleaned and disposed of. This, though, is very expensive and, unless a standby sand removal vessel is included in the seabed system, it results in a loss of production whilst the operation is carried out.

An alternative is to design equipment to withstand the abrasion of the sand. This requires specifying hard materials for all surfaces in contact with the production fluid containing sand and is especially expensive where long lengths of pipeline are required.

Production fluid may be conveyed to a host facility where the fluid mixture is separated into separate fluids in a separator vessel at the host facility. Sand in the production fluid mixture would settle in the separator vessel at the host facility and the sand would need to be regularly removed from the vessel. Furthermore, should there be pumps on the seabed, it is most unlikely that these pumps would be able to pump the production fluid without suffering premature failure as a result of the sand.

It is therefore an object of the present invention to provide a method and apparatus which overcomes at least some of the above-mentioned disadvantages.

According to one aspect of the present invention there is provided a method for collecting sand contained in production fluid and disposing of the collected sand, comprising the steps of:

flowing production fluid through sand removal means and collecting sand from the production fluid in the sand removal means;

supplying flushing fluid for flushing the sand removal means;

flushing sand from the sand removal means with the flushing fluid; and

directing the mixed flushed sand and flushing fluid away from production fluid processing means located downstream of the sand removal means.

By flushing sand from the sand removal means, the sand removal means is not required to be retrieved in order to remove the collected sand.

The method may include the step of pressure boosting the supplied flushing fluid before the flushing fluid enters the sand removal means.

The method may include commingling the mixed flushed sand and flushing fluid with production fluid from the production fluid processing means downstream of the production fluid processing means. Hence, the mixed flushed sand and flushing fluid is not directed through the production fluid processing means. Thus sand damaging the processing means is avoided. There may be the additional steps of conveying the commingled production fluid, flushed sand and flushing fluid to a host facility, and separating the flushed sand from the production fluid. The flushed sand may then be cleaned and disposed of.

There may be included the additional steps of cleaning the mixed flushed sand and flushing fluid directed away from the production fluid processing means, and disposing of the cleaned sand and cleaned flushing fluid. The cleaned sand and cleaned flushing fluid may be at least partially disposed of into the surrounding environment. The cleaned sand and cleaned flushing fluid may be at least partially disposed of into at least one cavity below the surface of ground underneath water. By disposing of the cleaned sand and cleaned flushing fluid in these ways, the need to transport sand back to the host facility is avoided, hence avoiding abrasive wear to the pipeline used for conveying fluid to the host facility.

The flushing fluid may be supplied from environmental water or water surrounding the sand removal means.

The flushing fluid may be supplied from a host facility. The flushing fluid may be supplied from the host facility via a reservoir. The flushing fluid from the reservoir may be pressure boosted by a pump. This enables the flushing fluid to be supplied from the host facility at a low pressure. Consequently, an expensive high pressure pipeline is not required between the host facility and the reservoir to supply the flushing fluid.

Desirably, the production fluid processing means comprises fluid separation means and the flushing fluid may be supplied from the fluid separation means wherein the flushing fluid comprises a fluid separated by the fluid separation means. The pressure in the fluid separation means may be sufficient for the flushing fluid to flush sand from the sand removal means. However, the flushing fluid may be pressure boosted before entering the sand removal means if necessary.

The method may include diverting at least some of the production fluid from downstream of the sand removal means, and flushing sand from the sand removal means with said diverted production fluid.

According to another aspect of the present invention there is provided an apparatus for collecting sand contained in production fluid and disposing of the collected sand, comprising:

sand removal means for removing sand from the production fluid;

flushing means for flushing sand from the sand removal means with a flushing fluid; and

means for directing the mixed flushed sand and flushing fluid away from production fluid processing means located downstream of said sand removal means.

The apparatus may include a pump for pressure boosting the flushing fluid before the flushing fluid enters the sand removal means.

Preferably, the apparatus includes said production fluid processing means. The production fluid processing means may comprise fluid separation means.

The apparatus may include commingling means downstream of the production fluid processing means for commingling production fluid processed by said processing means with the mixed flushed sand and flushing fluid.

The apparatus may have cleaning means for cleaning the mixed flushed sand and flushing fluid directed away from the production fluid processing means, before disposal of the sand and flushing fluid. There may be provided means for conveying the cleaned sand and flushing fluid into the environment surrounding the apparatus.

Conveniently, the apparatus has a reservoir for supplying the flushing fluid.

An outlet from the fluid separation means may be connected to the sand removal means to enable the flushing fluid to comprise a fluid separated by the fluid separation means.

The apparatus may include means for returning at least some of the production fluid downstream of the sand removal means into the sand removal means, wherein the flushing fluid comprises the returned production fluid.

There may be provided a seabed processing means comprising said apparatus including said production fluid processing means.

Preferably there is provided a retrievable production fluid processing module for a seabed processing system, the module comprising the apparatus described above including said production fluid processing means, and a fluid connection means for connection with the seabed system, whereby fluid to be processed by the production fluid processing means enters the module via the fluid connection means and fluid processed by the production fluid processing means leaves the module via the fluid connection means. The module may be of the general type forming part of the modular system designed by Alpha Thames Ltd of Essex, United Kingdom, and named Alpha PRIME.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:—

FIG. 1 is a schematic view of an embodiment of an apparatus for collecting sand contained in production fluid and disposing of the collected sand constructed in accordance with the invention; and

FIGS. 2 to 9 are schematic views of modified apparatus.

Referring to FIG. 1 of the accompanying drawings, a retrievable module 1 is illustrated for a modular seabed processing system (not shown) which is connected to a host facility by pipelines. The host facility may be, for example, onshore or on a fixed or a floating rig. The module is connected to a docking unit (not shown) of the system by a multi-ported connector 2 which is described in GB-A-2261271. The connector 2 has two complementary portions 3,4 of which one portion 3 forms part of the module and the other portion 4 is on the docking unit. The portions each have a plurality of bores 5,6,7 which are aligned when the two portions are engaged. Each bore has an associated isolation valve 8,9,10 (only the valves for the module portion 3 are shown) whereby all the valves are closed in both portions to isolate the bores when the two portions are to be disengaged.

A first bore 5 is connected to a first inlet 11 of a sand removal unit 12 by a conduit 13 containing the isolation valve 8 associated with the bore 5 and an actuation fail safe valve 14. A first outlet 15 from the sand removal unit 12 is connected to a first inlet 16 of a two-phase fluid separation chamber 17 by a conduit 18 containing an actuation pressure control valve 19. A first outlet 20 from the fluid separation chamber 17 has a vortex breaker 21. The first outlet 20 is connected to a second bore 6 of the fluid connector 2 by a first outlet conduit 22 containing a recirculation pump 23, an actuation flow control valve 24, a non-return valve 25 and the isolation valve 9 associated with the bore 6. A recirculation loop pipe 26 containing a flow restriction device 27, connects the first outlet conduit 22, downstream of the recirculation pump 23, to a second inlet 28 for the fluid separation chamber 17. A second outlet 29 from the chamber 17 is connected to a third bore 7 of the fluid connector 2 by a second outlet conduit 30 containing an actuation pressure control valve 31 and the isolation valve 10 associated with the bore 7.

A second outlet 32 from the sand removal unit 12 is connected by a flushing outlet conduit 33 containing a non-return valve 34 to a junction 90 on the first outlet conduit 22 downstream of the non-return valve 25.

The sand removal unit 12 has a second inlet 35 connected to a port on the outside surface of the retrievable module 1. The port has an inlet filter 37 and is connected to the second inlet 35 by a conduit 38 containing a suction pump 39.

The module 1 has a control unit 40 connected to and arranged to control the actuation valves 14,19,24,31 and the pumps 23,39. The control unit 40 is also connected to a sensor 41 for detecting the amount of sand in the sand removal unit 12 and a sensor 42 for detecting the position of an interface between a gas and a liquid in the fluid separation chamber 17.

The operation of the retrievable module 1 installed in a modular seabed processing system will be now described.

Production fluid mixture from a subsea hydrocarbon reservoir enters the module 1 via the first bore 5 and is passed through the sand removal unit 12 which removes sand from the mixture. The production fluid mixture then enters the fluid separation chamber 17 where it is separated into a gas and a mixture of oil and water. The oil and water mixture leaves the fluid separator chamber 17 via the first outlet 20 and exits from the module 1 via the second bore 6. Oil and water mixture leaving the chamber 17 may be directed back into the chamber 17 via the recirculation pipe 26 as a means of achieving fluid level control within the fluid separation chamber 17. The gas leaves the chamber 17 via the second outlet 29 and exits from the module 1 via the third bore 7.

When the sand sensor 41 has detected a sufficient amount of sand in the sand removal unit 12 to be removed, the control unit 40 operates the suction pump 39 to draw in seawater from outside the retrievable module 1 via the filter 37. The seawater is pressure boosted by the pump 39, to a pressure which enables the seawater to flush sand from the unit 12. The mixture of flushed sand and seawater is commingled with the mixture of oil and water in the first outlet conduit 22 at the junction 90 and the commingled mixture exits from the module 1 via the second bore 6 and is conveyed to a host facility. At the host facility, the sand is separated from the commingled mixture. The sand is then cleaned and is ready for disposal.

Various modifications to the retrievable module 1 will now be described in which parts which correspond to those shown in FIG. 1 are designated with the same reference numerals and are not described in detail below.

FIG. 2 illustrates one modification to the retrievable module 1. In the modified retrievable module 43, the second outlet 32 from the sand removal unit 12 is connected to a sand clean-up unit 44. A first outlet 45 from the sand clean-up unit 44 is connected to a port 46 on the outside of the module 43, and a second outlet 47 from the sand clean-up unit 44 is connected to the conduit 18 between the sand removal unit 12 and the fluid separation chamber 17 which may be either upstream or downstream of the pressure control valve 19. Thus, the mixture of flushed sand and seawater from the sand removal unit 12 has its hydrocarbons removed by the clean-up unit 44 and the clean sand and seawater are disposed into the sea outside the module 43 via the port 46. The hydrocarbons removed from the sand and seawater are recombined with the production fluid mixture in the conduit 18 downstream of the sand removal unit 12. In a modification to this, the cleaned sand and seawater are disposed of into a suitable cavity/reservoir beneath the seabed.

Another modification to the retrievable module 1 is shown in FIG. 3. The seabed processing system is connected to an umbilical 48 from the host facility which is arranged to convey fluid from the host facility. This fluid is arranged to enter the modified retrievable module 49 by a power/controls/fluids connector 50 which has two complementary engagable portions 51,52. One portion 51 forms part of the module 49 and the other portion 52 is secured to the subsea facility. The connector portion 51 is connected to an accumulator vessel 53 via a non-return valve 54 and the accumulator vessel 53 is connected to the second inlet 35 of the sand removal unit 12 by a conduit 56 containing an actuation flow control valve 57 connected to the control unit 40 of the module, and another non-return valve 58.

To store fluid in the accumulator vessel 53, water, or another suitable flushing fluid, is pressure boosted by a pump at the host facility and is conveyed by a small diameter feed pipe from the host facility to the subsea facility. The water is then fed via the power/controls/fluids connector 50 into the accumulator vessel 53 which acts as a reservoir.

To remove the sand collected in the sand removal unit 12, the control unit 40 operates the actuation flow control valve 57 so that it is sufficiently opened to enable a high enough flow rate of water to leave the accumulator vessel 53 and flush the sand. The flow control valve 57 may be adjusted by the control unit to regulate the volumetric flow rate of water and the non-return valve 58, downstream of the flow control valve 57, ensures that back flow is not experienced at the accumulator vessel 53. The mixture of flushed sand and water in the flushing outlet conduit 33 is commingled with the mixture of oil and water in the first outlet conduit 22 from the fluid separator chamber 17 and the commingled mixture exits from the module 49 via the second bore 6 where it is transported to a host facility.

FIG. 4 illustrates a modification to the retrievable module 49 shown in FIG. 3. In the modified retrievable module 60, the flow control valve downstream of the accumulator vessel 53 has been replaced by a pump 61 connected to the control unit 40 and the accumulator vessel 53 has an associated level sensor 62 which is also connected to the control unit 40. The level sensor 62 ensures that the pump 61 is switched off in the event that the level in the accumulator vessel 53 falls below that of the inlet to the outlet pipe 91 from the accumulator vessel 53 to the pump 61. In use, the control unit 40 operates the pump 61 so that the pump 61 draws flushing fluid from the accumulator vessel 53 and pressure boosts the fluid. The flushing fluid is then pumped into the sand removal unit 12 at a sufficient pressure so as to flush sand from the unit 12 and the mixture of flushed sand and water is at a sufficient pressure so as to be commingled with the mixture of oil and water.

The retrievable module 63 shown in FIG. 5 is another modification of the retrievable module 1 described above. In the modified module 63, the conduit 18 between the sand removal unit 12 and the fluid separator chamber 17 has an actuation flow control valve 64 connected to the control unit 40 and upstream of the actuation pressure control valve 19. A branch take-off pipe 65 containing another actuation flow control valve 66 connected to the control unit 40, connects the conduit 18 upstream of the flow control valve 64 to the second inlet 35 of the sand removal unit 12. By the control unit 40 operating the flow control valves 64,66, sand free production fluid from the sand removal unit 12 is routed back into the sand removal unit 12 via the branch take-off pipe 65 to flush sand collected there. The mixture of flushed sand and production fluid in the flushing outlet conduit 33 is then commingled with the oil and water mixture in the first outlet conduit 22 from the fluid separator chamber 17 and the commingled mixture exits from the module 63.

The retrievable module 70 shown in FIG. 6 is another modification of the retrievable module 1 described above. A flushing conduit 67 containing an actuation flow control valve 68 connected to the control unit 40, and a non-return valve 69, connects the recirculation loop pipe 26 upstream of the flow restriction device 27, to the second inlet 35 to the sand removal unit 12. To flush sand from the sand removal unit 12, the oil and water mixture leaving the first outlet of the fluid separator chamber is diverted from the first outlet conduit 22 by the control unit 40 routing the oil and water mixture into the sand removal unit 12 by adjusting the first outlet conduit flow control valve 24 downstream of the loop pipe 26 and the flow control valve 68 in conduit 67. The mixture of flushed sand and oil and water in the flushing outlet conduit 33 is then commingled with the oil and water mixture in the first outlet conduit 22 and the commingled mixture exits from the modified retrievable module 70.

FIG. 7 shows a modification to the retrievable module 70 shown in FIG. 6. In the modified retrievable module 71, there is no recirculation loop pipe 26 connected to the fluid separation chamber 17 and hence no pressure restricting device 27, nor is there a pump 23 on the first outlet conduit 22. This arrangement is suitable for the situation when the pressure in the two-phase fluid separator chamber 17 is sufficient to drive the separated gas and the separated oil and water mixture to their destinations.

Another modified retrievable module 72 is illustrated in FIG. 8 which differs from the initial retrievable module 1 described above essentially in that it has a three-phase fluid separator chamber 73 and does not use seawater surrounding the module to flush sand from the sand removal unit 12.

The three phase fluid separator chamber 73 has an additional third outlet 74 which is connected to a fourth bore 75 of the module portion 3 of the fluid connector 2 by a third outlet conduit 76 containing a pump 77, an actuation flow control valve 78 and an isolation valve 79 associated with the bore 75. The pump 77 and the flow control valve 78 are connected to the control unit 40. The third outlet 74 from the fluid separation chamber 73 has a vortex breaker 80. A recirculation loop pipe 84 containing a flow restriction device 85, connects the third outlet conduit 76, downstream of the pump 77, to a third inlet 86 of the fluid separation chamber 73 and a flushing conduit 81 containing an actuation flow control valve 82 connected to the control unit 40, and a non-return valve 83, connects the recirculation loop pipe 84 upstream of the flow restriction device 85 to the second inlet 35 of the sand removal unit 12.

The operation of the retrievable module 72 installed in a modular seabed processing system, where it differs from the operation of the retrievable module 1, will be now described.

The production fluid mixture is separated in the three-phase fluid separation chamber 73 into oil, gas and water. The oil and gas leave the chamber 73 via the first and second outlets 20,29 respectively and exit from the module 72 via the second and third bores 6,7 respectively. The water leaves the fluid separator chamber 73 via the third outlet 74 and exits from the module 72 via the fourth bore 75. Water leaving the chamber 73 may be directed back into the chamber via the recirculation pipe 84.

To remove the sand collected in the sand removal unit 12, the control unit 40 adjusts the two flow control valves 78,82 to direct water along the flushing conduit 81 and into the unit 12. The mixture of flushed sand and water in the flushing outlet conduit 33 from the unit 12 is commingled with the oil in the second outlet conduit 22 and the commingled mixture exits from the module via the second bore 6 where it is transported to the host facility.

The retrievable module may be configured for two-phase separation whereby the production fluid has water separated from it.

The retrievable module 87 shown in FIG. 9 is a modification of the retrievable module 72 illustrated in FIG. 8 in which the pump 77 and the recirculation pipe 84 connected to the third outlet from the fluid separation chamber 73 are not included. Furthermore, the pump 23 and the recirculation pipe 26 connected to the second outlet 20 from the fluid separation chamber 73 are not included. This arrangement is suitable for the situation when the pressure in the three-phase fluid separator chamber 73 is sufficient to drive the separated water and oil to their destinations.

The sand clean-up unit 44 shown in FIG. 2 is applicable to any flushing apparatus that utilizes water or any other suitable fluid as the flushing fluid.

Whilst the invention has been described in the context of a subsea hydrocarbon field, it would also be applicable to other areas such as swamps whereby the apparatus used would be land based. 

1. A method for collecting sand contained in production fluid and disposing of the collected sand, comprising the steps of: flowing production fluid through sand removal means (12) and collecting sand from the production fluid in the sand removal means; supplying flushing fluid for flushing the sand removal means (12); flushing sand from the sand removal means (12) with the flushing fluid; and directing the mixed flushed sand and flushing fluid away from production fluid processing means (17) located downstream of the sand removal means (12).
 2. A method as claimed in claim 1, including commingling the mixed flushed sand and flushing fluid with production fluid from the production fluid processing means (17) downstream of the production fluid processing means.
 3. A method as claimed in claim 2, including the additional steps of conveying the commingled production fluid, flushed sand and flushing fluid to a host facility, and separating the flushed sand from the production fluid.
 4. A method as claimed in claim 1, including the additional steps of cleaning the mixed flushed sand and flushing fluid directed away from the production fluid processing means (17), and disposing of the cleaned sand and cleaned flushing fluid.
 5. A method as claimed in claim 4, wherein the cleaned sand and cleaned flushing fluid are at least partially disposed of into the surrounding environment.
 6. A method as claimed in claim 5, wherein the cleaned sand and cleaned flushing fluid are at least partially disposed of into at least one cavity below the surface of ground underneath water.
 7. A method as claimed in claim 6, wherein the flushing fluid is supplied from environmental water or water surrounding the sand removal means (12).
 8. A method as claimed in claim 1, including diverting at least some of the production fluid from downstream of the sand removal means (12), and flushing sand from the sand removal means with said diverted production fluid.
 9. A method as claimed in claim 6, wherein the production fluid processing means comprises fluid separation means (17) and the flushing fluid is supplied from the fluid separation means (17) wherein the flushing fluid comprises a fluid separated by the fluid separation means (17).
 10. A method as claimed in claim 6, wherein the flushing fluid is supplied from a host facility.
 11. A method as claimed in claim 10, wherein the flushing fluid is supplied from the host facility via a reservoir (53).
 12. A method as claimed in claim 11, wherein the flushing fluid from the reservoir (53) is pressure boosted by a pump (61).
 13. A method as claimed in claim 13, including the step of pressure boosting the supplied flushing fluid before the flushing fluid enters the sand removal means (12).
 14. An apparatus for collecting sand contained in production fluid and disposing of the collected sand, comprising: sand removal means (12) for removing sand from the production fluid; flushing means (39) for flushing sand from the sand removal means (12) with a flushing fluid; and means for directing the mixed flushed sand and flushing fluid away from production fluid processing means (17) located downstream of said sand removal means (12).
 15. An apparatus as claimed in claim 14, including commingling means (90) downstream of the production fluid processing means (17) for commingling production fluid processed by said processing means with the mixed flushed sand and flushing fluid.
 16. An apparatus as claimed in claim 14, including cleaning means (44) for cleaning the mixed flushed sand and flushing fluid directed away from the production fluid processing means (17) before disposal of the sand and flushing fluid.
 17. An apparatus as claimed in claim 16, including means (45,46) for conveying the cleaned sand and flushing fluid into the environment surrounding the apparatus.
 18. An apparatus as claimed in claim 14, including a reservoir (53) for supplying the flushing fluid.
 19. An apparatus as claimed in claim 17, including means (64,65,66) for returning at least some of the production fluid downstream of the sand removal means (12) into the sand removal means, wherein the flushing fluid comprises the returned production fluid.
 20. An apparatus as claimed in claim 19, including said production fluid processing means (17).
 21. An apparatus as claimed in claim 20, wherein the production fluid processing means comprises fluid separation means (17).
 22. An apparatus as claimed in claim 21, including an outlet (20) from the fluid separation means (17) connected to the sand removal means (12) to enable the flushing fluid to comprise a fluid separated by the fluid separation means.
 23. An apparatus as claimed in claim 22, including a pump (39) for pressure boosting the flushing fluid before the flushing fluid enters the sand removal means (12).
 24. A retrievable production fluid processing module (1) for a seabed processing system, the module comprising the apparatus as claimed in claim 23, including a fluid connection means (2) for connection with the seabed system, wherein fluid to be processed by the production fluid processing means enters the module (1) via the fluid connection means (2) and fluid processed by the production fluid processing means (17) leaves the module via the fluid connection means (2). 